You should clone that repository into the tests subdirectory (your
clone must be named text-test-data locally), then run make -C
tests/text-test-data to uncompress the test files. Many tests and
benchmarks will fail if the test files are missing.

Functional tests

The functional tests are located in the tests subdirectory. An overview of
what's in that directory:

Makefile Has targets for common tasks
Tests Source files of the testing code
scripts Various utility scripts
text-tests.cabal Cabal file that compiles all benchmarks

The text-tests.cabal builds:

A copy of the text library, sharing the source code, but exposing all internal
modules, for testing purposes

The different test suites

To compile, run all tests, and generate a coverage report, simply use make.

Benchmarks

The benchmarks are located in the benchmarks subdirectory. An overview of
what's in that directory:

Makefile Has targets for common tasks
haskell Source files of the haskell benchmarks
python Python implementations of some benchmarks
ruby Ruby implementations of some benchmarks
text-benchmarks.cabal Cabal file which compiles all benchmarks

To compile the benchmarks, navigate to the benchmarks subdirectory and run
cabal configure && cabal build. Then, you can run the benchmarks using:

./dist/build/text-benchmarks/text-benchmarks

However, since there's quite a lot of benchmarks, you usually don't want to
run them all. Instead, use the -l flag to get a list of benchmarks:

./dist/build/text-benchmarks/text-benchmarks

And run the ones you want to inspect. If you want to configure the benchmarks
further, the exact parameters can be changed in Benchmarks.hs.